HTTPS Request pending with ServiceWorker (iFrame)

I have a web app which loads the serviceWorker in production mode. The app itself gets loaded within an iframe. I noticed that HTTPS call remain in status “Pending” (please see screenshot below) and time out after ca. 5 minutes. When I deactivate the service worker everything works fine.

Does anyone know what the problem is and how to fix this?

Thank you very much!